1. Field of the Invention
The present invention generally relates to a radio telecommunication system for use in a radio telecommunication apparatus such as a portable telecommunication apparatus or a mobile telecommunication apparatus, and more particularly to a radio telecommunication system capable of maintaining the quality of signals received by the radio telecommunication apparatus.
2. Description of the Related Art
FIG. 18 shows a block diagram illustrating a digital radio telecommunication system. The system includes a control station CS connected to a landline network NW. The control station CS is further connected to plural base stations BS1, BS2, BS3 . . . over landlines CL1, CL2, CL3, . . . . Each base station has its own radio zone E1, E2, E3, . . . . Each portable apparatus communicates over radio channels with the base station in the radio zone in which it is located. Further each portable apparatus is connected to the landline network NW through the base station BS and the control station CS.
This system adopts a time division multiple access system, referred to as a TDMA system, as a communication system. In the TDMA system, each radio channel for a downward channel and for an upward channel includes a plurality of time frames. Each frame includes six time slots. In the event that a communication link is formed between the base station and the portable station, one time slot for the downward signal is assigned from the downward radio channel and one time slot for the upward signal is assigned from the upward radio channel. After that, the portable apparatus receives the downward signal in the assigned downward slot and sends the upward signal in the assigned upward slot. FIG. 18 shows downward slots in each downward channel and upward slots in each upward channel. Referring to FIG. 18, if the portable apparatus PS is assigned TS1 as a downward slot and assigned TS1' as an upward slot, the portable apparatus PS sends the intermittent upward signal to the base station BS during a period corresponding to the upward slot TS1'. Furthermore, the portable apparatus PS receives the downward signal during a period corresponding to a downward slot TS1. Thus, the upward slot TS1' is defined as a transmit slot and the downward slot TS1 is defined a receive slot. During a period corresponding to the downward slots TS2, TS3, TS4, TS5, the apparatus PS sends no signal to the base station BS. Therefore, the slots corresponding to the slots TS2, TS3, TS4, TS5 are defined as an idle slot. The above six slots TS1.about.TS6 repeat in every frame. However, if an obstacle such as a tall building is located between the base station and the portable apparatus, the quality of signals received by the portable apparatus becomes low. As a result, it is difficult for the user to listen to the decoded speech sound. This difficulty occurs when the signal is reflected from other buildings and the portable apparatus receives the reflected signal.